In order to harden the raceways of bearing rings, in particular of large rolling-element bearings, an induction hardening method is usually used these days. In this method, the raceways are heated by induction using an inductor and then quenched by a cooling liquid flowing from a quenching sprinkler. Here in particular with large bearing rings the raceway is selectively heated in the partial region of the inductor to a temperature above the austenitizing temperature and quickly cooled by the quenching sprinkler to a temperature below the martensite start temperature so that a transformation into an austenite structure occurs in an outer layer of the raceway, while in an internal region the initial microstructure remains. It can thereby be achieved that the raceway can be subjected to high mechanical stresses; simultaneously, however, it can be ensured by the non-austenitized internal region that the bearing ring retains its predetermined ductility and has sufficient stability with respect to crack formation.
In order to achieve this selective heating, inductors are used that are moved relative to the workpiece, in particular the bearing ring. Here either the inductor or the workpiece can be moved. Furthermore for an optimal result the inductors are adapted to the contour of the to-be-treated workpiece.
Here an inductor usually comprises an induction coil for heating the workpiece and a quenching sprinkler with which the heated region is quenched. As mentioned above a cooling liquid is used for this purpose, which quickly brings the temperature in the heated region from the austenitizing temperature to a temperature below the martensite start temperature, whereby the martensite transformation process is completed to the greatest possible extent. However, care must be taken that during operation of the inductor the liquid cooling medium does not reach the heating zone of the induction coil, since otherwise due to the temperature fluctuation caused by evaporation increased crack formation can occur or undesirable structural transformations can arise.
It has therefore been proposed, as described, for example, in DE 10 2010 002 531, to atomize the liquid cooling medium using a cooling gas to a cooling mist that instantly evaporates on the heated workpiece and thereby can no longer condense on the workpiece as a liquid.
However, it is disadvantageous in this prior art that the use of a cooling mist is not possible with inductors that are moved relative to the workpiece, since the cooling mist can also settle in an uncontrolled manner on the surfaces actually to be heated. The heating zone is thereby strongly cooled such that significantly more energy must be supplied to the inductors in order to reach the required temperature above the austenitizing temperature in the heating zone of the workpiece.
Alternatively it is also known to use a sealing-air (blocking-air) shower between induction coil and quenching sprinkler that should provide a sealing-air curtain that should ensure a limiting of cooling-liquid entry into the heating zone.
However, with the devices including sealing air it has been shown that the quenching sprinkler and the induction coil must be separated very far from each other so that the sealing air can develop a sufficient effect. Thus the time between heating and sudden cooling becomes too long for an optimal hardening process.